Diaphragm

Glossary Terms

an 8-bladed iris diaphragm image by Dusty Medusa (Image rights)

A diaphragm (or iris or iris diaphragm) is a mechanism in a camera that makes a variable aperture to control the intensity of light that passes through the lens. Along with shutter speed, this is what controls the exposure received by the film or image sensor.

A diaphragm may take many forms, from very simple devices for "point-and-shoot" film cameras consisting of just two notched pieces of metal, to more complex ones used in higher-quality cameras which have many blades arranged in a circle. This arrangement, also called an "iris" after the corresponding structure in the eye, creates a nearly-circular aperture whose size can be varied as needed. There may be as few as 5 blades to as many as 19. In cameras with a small number of diaphragm blades, the shape of the aperture itself (e.g. a hexagon) can often be seen in defocused points of light.

To transmit a particular light intensity, the physical diameter of the aperture must scale up with increasing lens focal length. Thus, apertures are expressed by the f-stop scale or "F-ratio," which is the focal length divided by the entrance pupil diameter provided by the diaphragm opening.

Common Diaphragm Types

These are listed in historical order from oldest to newest.

• Waterhouse Stops - An aperture of a specific size is cut in a metal plate. To change the aperture size, the plate must be removed and replaced with a plate having a different sized aperture hole.

• Manual - A multi-bladed iris with an aperture that can be adjusted by some type of control on the outside of the lens or camera. The size adjustment is continuous and the user must rely on markings to position the control at a specific aperture value.

• Preset - An improvement on the manual diaphragm that includes detents or preset locking points in the aperture control at commonly used aperture sizes such as f/2.8 or f/5.6, making it easy for the user to set a precise aperture size.

• Automatic - An improvement on the preset diaphragm that includes the ability to hold the aperture at the maximum possible size while composing the shot. When the shutter release is triggered, the aperture quickly closes to the aperture size preset by the user.